Piston driven internal combustion engines commonly have valvetrains that affect flow into and out of a cylinder. Intake and exhaust valve opening and closing positions, relative to crankshaft position, can be at fixed positions, or alternatively, at positions that vary with engine operating conditions. Typically, for engines having adjustable valve timing, engine-operating conditions are used as a basis to adjust valve timing.
One method to adjust engine valve timing is described in U.S. Pat. No. 6,681,741. During engine starting, this method retards intake valve opening timing to after exhaust valve closing while also retarding fuel injection timing. The valve retarding operation specifically operates the intake and exhaust valve timing to form a negative overlap condition, i.e., the intake and exhaust valves are not concurrently in an open position during the intake stroke of a cylinder cycle. Such a combination is used in an attempt to increase air-fuel mixing, reduce fuel adhering to intake port walls, and improve combustion stability.
While it may be beneficial to attempt to improve air-fuel mixing, reduce fuel adhering to port walls, and improve combustion stability, the inventors herein have recognized that the use of negative valve overlap may increase engine oxides of nitrogen (NOx) and hydrocarbon (HC) emissions by limiting the amount of internal EGR (also referred to as residual gases), i.e., the amount of exhaust gas trapped in the cylinder from the last combustion event. When an incoming cylinder air-fuel charge is not diluted by residual gases, combustion temperatures can increase and an opportunity may be lost to combust hydrocarbons that may not have combusted during the previous combustion event. This can also increase part load fuel consumption and emissions.
Another known method to adjust engine valve timing of an engine with adjustable intake and exhaust valve timing, appears to use advanced intake valve opening timing from before top-dead-center (TDC) of the cylinder intake stroke to approximately 65° before TDC of the cylinder exhaust stroke (65° degrees of advance). Furthermore, fuel is injected while the intake valve is open, during a first set of conditions, and fuel is injected while the intake valve is closed, during a second set of conditions.
While open valve injection with advanced timing may provide for increased fuel and air mixing, the inventors herein have recognized that it may also come at a detriment to fuel economy. Namely, advanced intake valve timing and open valve fuel injection can have higher engine pumping work and higher part load fuel consumption than desired because of the relatively advanced valve timing. During early intake valve closing, less fresh charge is pushed back into the intake port during the first part of the compression stroke. Consequently, a given engine load may be maintained at a relatively low manifold pressure, and may produce more than a desired amount of engine pumping work. In addition, due to limited charge cooling that may be available from the above-mentioned valve timing and fuel injection strategy; the method may provide less than desired fuel economy and may result in engine knocking at higher spark angles.